This invention relates generally to an imaging method for the manufacture of microdevices. More particularly, in one aspect, the invention is concerned with an imaging method or an illumination method therefor, suitably usable in forming on a workpiece a fine pattern of a linewidth of 0.5 micron or less.
The increase in the degree of integration of a semiconductor device has been accelerated more and more and, along a such trend, the fine processing techniques have been improved considerably. Particularly, the optical processing technique which is a major one of them has been advanced to a level of submicron region, with the start of a 1 mega DRAM. A representative optical processing machine is a reduction projection exposure apparatus, called a xe2x80x9cstepperxe2x80x9d. It is not too much to say that enhancement of resolution of this apparatus determines the future of the semiconductor device.
Conventionally, the enhancement of resolution of the stepper mainly relies on enlarging the N.A. (numerical aperture) of an optical system (reduction projection lens system). Since, however, the depth of focus of an optical system is in inverse proportion to the square of the N.A., the enlargement of the N.A. causes an inconvenience of decreased depth of focus. In consideration of this, attempts have been made recently to change the wavelength of light for exposure, from the g-line to the i-line or to excimer laser light of a wavelength not longer than 300 nm. This aims at an effect that the depth of focus and the resolution of an optical system can be improved in inverse proportion to the wavelength. On the other hand, in a way separate from shortening the exposure wavelength, a method using a phase shift mask has been proposed as a measure for improving the resolution. According to this method, a thin film is formed in a portion of a light transmitting area of a mask, which film serves to provide a phase shift of 180 deg. with respect to the other portion. The resolution RP of a stepper can be represented by an equation RP=k1xcex/N.A., and usually the stepper has a k1 factor of a level of 0.7-0.8. With the method using such a phase shift mask, the level of the k1 factor can be improved to about 0.35.
However, there remain many problems to realize such a phase shift mask method. Unsolved problems currently remaining are such as follows:
(1) A satisfactory thin film forming technique for forming a phase shift film has not yet been established.
(2) A satisfactory CAD (computer-aided designing) for design of a circuit pattern with a phase shift film has not yet developed.
(3) Depending on a pattern, a phase shift film cannot be applied thereto.
(4) With respect to the inspection and correction of a phase shift film, a satisfactory technique has not yet been established.
As stated, there remain many problems to realize a phase shift mask method.
It is an object of the present invention to provide a unique and improved imaging method suitable for the manufacture of microdevices such as semiconductor microcircuit devices.
It is another object of the present invention to provide a microdevice manufacturing method which uses such an imaging method.
It is a further object of the present invention to provide an exposure apparatus for the manufacture of microdevices, which uses such an imaging method.
In accordance with a first aspect of the present invention, there is provided an imaging method for imaging a fine pattern having linear features extending along orthogonal first and second directions, characterized by: providing a light source having decreased intensity portions at a center thereof and on first and second axes defined to intersect with each other at the center and defined along the first and second directions, respectively; and illuminating the pattern with light from the light source.
In accordance with a second aspect of the present invention, there is provided a method of imaging a fine pattern having linear features extending in orthogonal first and second directions, wherein the pattern is illuminated with light obliquely with respect to the pattern, the improvements residing in that: the strength of illumination in a predetermined plane of incidence is made greater than that in a first plane of incidence including the first direction and that in a second plane of incidence including the second direction and intersecting with the first plane of incidence perpendicularly.
In accordance with a third aspect of the present invention, there is provided a method of imaging a fine pattern having linear features each extending in a predetermined direction, wherein the pattern is illuminated with light obliquely with respect to the pattern, the improvements residing in that the illumination of the pattern with light along a path in a plane of incidence including the predetermined direction is substantially blocked; and the pattern is illuminated with light along a pair of paths which are symmetrical with each other with respect to the plane of incidence.
In accordance with a fourth aspect of the present invention, there is provided an illumination method in image projection, for illuminating a fine pattern of an original, characterized by: providing a light intensity distribution having decreased intensity portions at a center thereof and on first and second orthogonal axes with respect to which the original is to be placed.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.